These air spaces made the skulls light but strong and could have helped dinosaurs breathe, communicate, and hunt.

The extra room may even have paved the way for flight in some species.

"Air is a neglected system that is actually an important contributor to what animals do," said study co-author Lawrence Witmer, a paleontologist at Ohio University in Athens.

The research is detailed in a recent issue of the journal The Anatomical Record.

Empty-Headed

Witmer and colleague Ryan Ridgely made detailed CT scans of air cavities in the skulls of two predators, T. rex and Majungasaurus; and two ankylosaurs, Panoplosaurus and Euoplocephalus, both plant-eaters with armored bodies and short snouts. (See an illustration of another ankylosaur species.)

The results mark the first time scientists were able to accurately estimate the weight of a dinosaur's head.

A T. rex head, for example, would have weighed more than 1,100 pounds (about 500 kilograms), close to the average weight of an adult cow, Witmer and colleagues found.

Until now, paleontologists had to make do with estimates for the weight of dinosaur heads, said Tom Holtz, a paleontologist at the University of Maryland who was not involved in the research.

"Larry's team is able to calculate a volume for the skull, so they can constrain the weight far more securely," Holtz said. "This is the next best thing to having a fleshy T. rex head to dissect."